Revert "Remove the usage of instance['extra_specs'

This reverts commit 76c47c1c3057d5ee479cf1e55b803f5ac9faba1d.

Fix libvirt vif get_config interface breakage.

In 76c47c1 we added a new arg to the libvirt vif driver get_config
interface (instance_type=None). We did not update all the libvirt
drivers to use it however which can break things badly (instances
fail to spawn, etc). We clearly need more test coverage here as well.

This should fix SmokeStack.

Fixes LP Bug #1155950.

Change-Id: I073d119ea2bfd5c314d2d8c8bbf2f6d7d649753a
Merge "Revert "Remove the usage of instance['extra_specs'"